1. Field of the Invention
The present invention generally relates to a delay locked loop (DLL), and more particularly, to a delay locked loop having adjustable unit delay elements.
2. Description of Related Art
For the operation of a DDR SDRAM (double data rate synchronous DRAM), a plurality of reference clock signals with a same frequency but different phases are used. A delay locked loop is used to lock the input reference clock signal and to generate a plurality of output clock signals with a same frequency but phases different from that of the input reference clock signal.
FIG. 1 is a block diagram of a conventional delay locked loop. Referring to FIG. 1, a conventional delay locked loop 10 mainly includes a phase detector 11, a delay line 12 and a multiplexer 13. The delay line 12 includes plural stages of unit delay elements connected in series. The output signal of a preceding stage of unit delay element would be delayed by the following stage of unit delay element. For simplicity, the delay line 12 herein includes, for example, four stages of unit delay element 12-1˜12-4. The amount of delay of each unit delay element, 12-1, 12-2, 12-3 and 12-4, which is termed as unit delay amount or unit delay, is fixed with the prior art and denoted by tUD. If the period of an input signal IN is tCK, when the delay locked loop is locked, we have tCK=n*tUD where n denotes number of stage, i.e. the number of the unit delay elements composing the delay line 12. The number of stages and the unit delay elements would determine the operation range of the delay locked loop.
All output signals of the unit delay elements 12-1˜12-4 are input to the multiplexer 13. The multiplexer 13 then selects one or more delay stages among the unit delay elements 12-1˜12-4 according to the output of the phase detector 11 to generate an output signal OUT. For example, the multiplexer 13 can select a single delay stage, and thus the input signal IN is delayed by the unit delay element 12-1 into the output signal OUT; the multiplexer 13 can select two delay stages, and thus the input signal IN is delayed by the unit delay elements 12-1 and 12-2 into the output signal OUT; the multiplexer 13 can also select a four delay stages, and thus the input signal IN is delayed by the unit delay elements 12-1˜12-4 into the output signal OUT.
The phase detector 11 is used to compare the input signal IN with the output signal OUT to get a phase difference and output the comparison result to the multiplexer 13.
With a higher and higher frequency of a clock signal (i.e. a shorter and shorter signal period) today, the corresponding unit delay tUD requires to be less and less, which would increasingly restrict the operation range of a delay locked loop. In addition, considering a unit delay provided by the conventional unit delay element is fixed in the prior art, in order to increase the operation range it can be achieved only by increasing the number of stages.
Based on the above-mentioned situation, an improved delay locked loop is really expected by the relevant manufactures so as to overcome the disadvantages of the prior art.